1. Field of the Invention
This invention relates to apparatus and methods for the implantation and subsequent testing of the acetabular cup portion of an artificial hip joint. More particularly, the invention relates to improved apparatus and methods for reliably implanting and testing the strength of the implant of the acetabular cup component of a hip joint.
2. Relevant Technology
The hip is one of the most versatile joints of the human body and serves an essential function in allowing an individual to lead a normal life. The human hip joint performs its function much better than many devices heretofore designed by human engineers, and can withstand forces which are not readily apparent to those unfamiliar with orthopedics and kinesiology. For example, during ordinary walking, the hip joint is routinely subjected to dynamic forces nearly four times greater than a person's body weight. The dynamic forces on the hip joint may be as great as ten times the person's body weight during activities such as running or jumping.
When functioning properly, the bones of the hip joint move together with very little friction. To function properly, a healthy hip joint requires an intact layer of hyaline cartilage, the material which makes up the articular cartilage on the opposing surfaces of the joint. Also, the bones of the joint must be in proper alignment and the synovial membranes must produce suitable amounts of lubricating (synovial) fluid. Furthermore, the joint structures must prevent the bones from being placed in an abnormal position.
The human hip joint is shown in FIG. 1, which is an anterior (that is, taken from the front of the body) cross-sectional view of the human hip. The semi-circular shape of the acetabulum 10 can be seen. The upper leg bone, or femur 12, which can be seen just below the ilium 14, is the longest and strongest bone in the body. The upper end of the femur is provided with a spheroidally shaped head 16, a neck 18, a greater trochanter 20, and a lesser trochanter 22. The femoral head 16 is shown in FIG. 1 in a "normal" relationship with the acetabulum 10. It should be appreciated that FIG. 1 is not intended to show the exact structures of the hip joint, since the structure varies somewhat from individual to individual, but to show the relationship of the major components which make up the hip.
FIG. 1 also illustrates the acetabulum articular cartilage 24 and femoral head articular cartilage 26. A space 28 is shown extending between the entirety of the two articular cartilage surfaces. This "joint space" 28 may or may not be present in a particular hip, depending upon the condition of the hip. Normally, the articular cartilage 24 is smooth and intact. When the articular cartilage 24 is damaged, however, pain and an accompanying restriction of motion can often result.
The femur 12 is provided with a femoral neck 18 which may be up to 5 cm long. The femoral neck 18 separates the shaft of the femur 12 from the femoral head 16. This arrangement allows the femur a substantial degree of movement without interference from the bones making up the pelvis.
The greater trochanter 20 serves as an attachment point for various muscles and ligaments. The lesser trochanter 20 serves a similar purpose. Round ligament 30 is thought to provide a passage for the blood vessels to the femoral head 16 and also to assist with spreading of synovial fluid over the articular cartilage surfaces 24, 26 to lubricate and nourish the cartilage.
The versatility of the hip joint can be appreciated by realizing that the normal range of motion includes flexion and extension (rotation in forward and rearward directions, respectively) and adduction and abduction (motion towards the center line of the body and away from the center line of the body, respectively).
While the hip joint generally serves its purpose very well, various disorders of the hip can cause a great deal of pain and loss of mobility and function. Some hip disorders are congenital, that is, they are present at birth. Other disorders of the hip are brought on by bacterial infection, which can occur at any age. Perhaps the most widespread disorder of the hip is arthritis. The term "arthritis" is generally used as a common name for the effects of several degenerative hip disorders.
Of the various types of arthritis, osteoarthritis is perhaps the most common. Osteoarthritis is a degenerative "wear and tear" process that affects substantial numbers of people. The final result of unchecked osteoarthritis is damaged articular cartilage which, in many cases, causes extreme pain as the damaged surfaces are rubbed together during joint movement. It has been estimated that between 8% and 15% of the population of developed countries, with higher percentages prevalent in older populations, suffer some degree of osteoarthritis.
One disorder of the hip which appears to lead to osteoarthritis is known as "congruence." Congruence occurs when the shape of the femoral head and the shape of the acetabular socket become matched so that the dome area of the acetabulum 10 and the femoral head 16 are nearly always in contact. Congruence of the hip can cause increased wear on the joint surfaces. Several of these disorders and other conditions are explained in more detail hereinbelow. Osteoarthritis may also involve the development of abnormal bony outgrowths on the joint surfaces known as osteophytes. An osteophyte consists of a lump of "cancellous" tissue (tissue having a lattice structure similar to the spongy tissue of the bone) which is capped by a sheet of soft tissue. Commonly, cysts also form on the femoral head and in the acetabulum with the hip joint. These cysts are often formed just under the articular cartilage and result in a great deal of pain.
Generally, osteoarthritis affects people past the age of 60 years without providing an easily recognizable, single cause. However, it may also develop in younger people, usually due to a congenital condition or disease. Furthermore, traumatic injury may cause the development of an osteoarthritic condition.
Another hip disorder is osteonecrosis, or death of a portion of a bone, which is due to an insufficient blood supply to part of, or the whole of, a bone. Osteonecrosis may be brought on by excessive alcohol consumption, administration of particular drugs, old age, or as a result of osteoarthritis.
In the prior art, several methods have been used for alleviating the pain and improving the function of a hip joint afflicted with a degenerative disorder such as osteoarthritis. Perhaps the earliest surgical procedure used to reduce pain due to a disorder of a hip joint was "ankylosing," or fusing the joint. This alternative, generally called "arthrodesis," alleviates pain in a diseased hip joint but also prohibits proper function of the joint. Thus, arthrodesis is generally not an acceptable procedure of relieving hip pain for most patients. In fact, hip surgery is quite often carried out in order to remedy a hip which has become ankylosed, stiff, or immovable.
In some other cases, "debridement" of a hip joint may be helpful. Debridement of the joint usually consists of removing unwanted bony spurs and loose pieces of bone and cartilage within the joint cavity. While this procedure is helpful in some cases, the most common cause of pain and loss of function is due to degeneration of the hip joint rather than abnormal growths or debris in the joint.
Osteotomy, which generally refers to the cutting and resetting of a bone, has also been used in an attempt to alleviate pain and restore function of the hip joint. By cutting and resetting the femur, for example, it may be possible to reorient the femur head 16 within the acetabulum 10 such that portions of the femur head 16 not affected by the degenerative disorder are used as weight-bearing surfaces. However, in the case of osteoarthritis, the surfaces of both the acetabulum 10 and the femur head 16 are generally involved in the degenerative condition. If the surface of the acetabulum 10 has been damaged, repositioning of the femoral head 16 will probably not provide relief.
Because of limitations of the foregoing procedures, one of the most common procedures used in treatment of hip disorders is the implantation of artificial joint components. This procedure is known as "arthroplasty," and has been one of the major areas of advancement in hip surgery during the past quarter century. Hip arthroplasty has included techniques known as interpositional arthroplasty, partial arthroplasty, and total arthroplasty.
Interpositional arthroplasty of the hip joint generally involves interposing a layer of material between the two opposing articular surfaces of the joint. For example, materials such as muscle, fibrous tissue, celluloid, silver plates, rubber sheets, magnesium, zinc, decalcified bones, and pig's bladder have all been used in interpositional arthroplasty of various joints. Cup-shaped structures made from gold foil, glass, or VITALLIUM.RTM. (a cobalt-chromium alloy) have also been interposed between the femoral head 16 and the acetabulum 10. Even further attempts have been made to encase the femoral head 16 within a metallic shell and also line the acetabulum 10 with a cup comprised of a plastic-like material.
Partial arthroplasty involves the replacement of one of the two opposing articular joint surfaces 24, 26. For example, this procedure is used where the femoral head 16 has been damaged but the acetabulum 10 is otherwise normal. In such a case, it may be beneficial to replace the femoral head 16 with an artificial prosthesis which will work in conjunction with the natural acetabulum 10. Partial arthro-plasty procedures have met with only limited success.
The most common arthroplasty procedure used to alleviate pain and restore hip function is total hip arthroplasty, also called a total hip replacement. While many different styles of hip replacement prosthesis have been implanted in patients, they generally resemble the prosthesis illustrated in FIG. 2. FIG. 2 also illustrates the femur 12 and a portion of the pelvis in cross-section in order to best show how the components of a total hip replacement are implanted in the body.
FIG. 3 is an exploded view of the main components of a prosthetic hip implant. These include an acetabular structure 50, including an outer acetabular cup 52, which often has a plurality of holes 54. The outer acetabular cup fits into an acetabular recess 51 within the pelvic bone 53, which is formed by removing enough of the surrounding bone and other tissue in the acetabular region of the pelvic bone 53 to create a conforming fit. An inner acetabular cup 56 fits inside the outer acetabular cup 52 and is held in place by means known in the art. The inner acetabular cup 56 also includes a generally hemispherical space 58 into which an artificial femoral head is placed.
FIG. 3 also shows a femoral structure 60, including a femoral head 62 connected to a femoral neck 64 and a stem 66, which is inserted into a hollowed recess of the femur 12.
The outer acetabular cup is generally made of a durable metal, while the inner acetabular cup is made of a smooth yet durable plastic, such as polyethylene. The femoral head 62 is generally made of a hard, durable metal with a smooth surface to interface with the polyethylene surface of the inner acetabular cup 56.
Conventional total hip replacement involves a complete internal amputation of the hip joint as suggested in FIG. 2. The conventional surgical procedure used during total hip replacement involves making a surgical incision to provide an approach to the hip. Once the hip is exposed, the joint is dislocated so that the femoral head 16 and acetabular socket 10 can be accessed. The femoral head 16 and neck 18 are then amputated. Often, the greater trochanter 20 is removed and reattached at a lower point by the use of wires 32. Once the femoral head 16 and neck 18 have been removed, the femoral canal (the central core of the bone, generally indicated as 34) is reamed so as to provide a cavity into which a stem 36 of a femoral component 38 may be inserted. The femoral canal 34 is reamed so that its diameter is significantly larger than the diameter of the femoral component stem 36.
One commonly accepted method of fixing the femoral component 38 to the femur 12 is by polymethylmethacrylate (PMMA), which is a two-component acrylic cement that has the advantage of exhibiting a rapid setting time. After mixing the two components, the femoral canal 34 is "packed" with unset PMMA. The stem 36 of the femoral component 38 is then inserted into the femoral canal 34 and the femoral component 38 is held in the proper position until the PMMA has set. Since the femoral canal 34 has been reamed out to a larger diameter than the stem 36 of the femoral component 38, the PMMA cement serves as a "grout" 37 which interfaces between the stem 36 and the remaining bone.
The femoral component 38 is available in varying sizes and styles, but nearly all of those presently used include a stem 36, a neck 40, and a ball-shaped head 42 similar to the analogous components of an actual femur 12 pictured in FIG. 1. Most of the prosthetic femoral components which are presently used are fabricated from a cobalt-chromium steel alloy or a titanium alloy.
Implantation of the acetabular components 44 also requires significant alteration of the bone structure. The acetabulum 10 is first reamed out to provide a cup-shaped cavity into which an outer acetabular cup 46 will be fixed. An inner acetabular cup 48 fits within the outer acetabular cup 46. Conventional acetabular components 44 used in total hip replacements are relatively large. Presently, most inner acetabular cups 48 are fabricated from ultra-high molecular weight polyethylene (UHMWP). The outer acetabular cup 46 is fixed within the reamed out cavity by PMMA adhesive, which once again serves as a grouting cement. The inner acetabular cup 48 is press-fitted within the outer acetabular cup 46.
After the femoral component 38 and the acetabular components 44 have been implanted, the greater trochanter 20, if previously removed, is reattached using the wires 32 at a point lower on the femur 12 so as to provide a mechanical advantage more favorable to the total hip prosthesis. The joint is then reduced and the surgical incision closed.
As mentioned previously, the stresses on the hip joint during ordinary activities are very high. During strenuous activities those stresses are increased several fold. These high stresses result in several mechanical difficulties in a patient fitted with a conventional total hip prosthesis. For example, it is not uncommon for the femoral component 38 to become dislocated from the acetabular components 44. Alternatively, the components may fail (i.e., fracture or break) due to the stresses placed upon them. A common difficulty is loosening of the components from the surrounding bone. Generally, problems such as loosening or failure are particularly acute with the femoral component. Nevertheless, problems with implanted prosthetic acetabular cups have developed over time which were not apparent at first.
For example, where bone cement has been used, there have been problems with the resorption of the surrounding bone next to the bone cement. This is because the body recognizes the bone cement as a foreign body, causing the body's immune system to attack the surrounding area, including the bone. When this occurs, the prosthetic acetabular cup connection becomes very weak and can fall out.
Another method of stabilizing acetabular cup implants involves the use of surgical screws, which are screwed into underlying bone through holes in the prosthetic acetabular cup. While a generally secure way of fastening the prosthetic acetabular cup, surgical screws can introduce infection into the patient's tissue, irritate and damage nerves, and rupture blood vessels, causing pain and other damage to the surrounding tissue. Similar problems occur with the use of surgical pegs, which generally require the drilling of holes into the surrounding bone area.
An example of an acetabular cup using pegs to stabilize an acetabular cup implant is found in U.S. Pat. No. 5,127,920 to MacArthur. A similar configuration can be found in U.S. Pat. No. 5,108,448 to Gautier, wherein a cup with numerous spike-like protrusions is inserted into the acetabular area. While these cups undoubtedly result in a fairly reliable fit, at least in a rotational sense, they possess the various problems associated with using spikes or pegs.
More recently, surgeons have begun to perform less complicated prosthetic acetabular cup implants by press-fitting the prosthetic acetabular cup into a slightly smaller recess within the acetabular area of the pelvic bone. The results have been mixed. If a cup implant is not sufficiently tightly fit, it is less likely to stay in place. Moreover, simply and blindly making a tighter fit has many risks, including the possibility of fracturing the pelvis bone, or the problem of "rim fitting."
In the case of rim fitting, the implanted cup makes tight contact with the outer rim of the recess but fails to make abutting contact with the lower portion of the recess. Thus, a rim fit may appear adequate although difficulties may appear due to the inability of bone to grow across the gap where an implanted acetabular cup has failed to "bottom out" against the surface of the lower portion of the recess.
The safest and least complicated prosthetic acetabular cup implant is one which is relatively round but which has a roughened or irregular surface. If the cup is adequately press fitted, surrounding bone will grow into the irregularities in the surface of the cup, resulting in a very strong and secure acetabular cup implant.
Examples of devices which are used to implant prosthetic acetabular cup implants are disclosed in U.S. Pat. No. 5,061,270 to Aboczki, U.S. Pat. No. 5,037,424 to Aboczki, U.S. Pat. No. 5,030,221 to Buechel et al., and U.S. Pat. No. 4,994,064 to Aboczki. Each of these implantation devices is made of rigid materials, are capable of imparting a strong implantation force to drive the cup implant into a recess in the acetabular area, and are able to position the cup so that the implanted cup is in the proper orientation. Nevertheless, none of these devices gives the doctor any indication as to the sufficiency or tightness of the acetabular cup implant. Thus, although each of these devices can often result in successful cup implants, there remains a certain level of uncertainty regarding the reliability of the implant.
Once total hip replacement has occurred, revision of the procedure (revision being the term used to describe when the prosthesis is replaced) can be extremely due to the amount of bone mass which was removed during the original procedure, as well as the extreme invasiveness of the procedure. If there were a reliable way to test the stability of the implant, the surgeon would then be able to take remedial measures such as using secondary methods of securing the implanted cup before sewing the patient up.
From the foregoing, it will be appreciated that what are needed are apparatus and methods for reliably implanting an acetabular cup that will result in a reliable implant, but which avoids the complications associated with using bone cement.
It will also be appreciated that it would be an improvement over the prior art to provide apparatus and methods for reliably implanting an acetabular cup that will result in a reliable implant, but which avoids the complications associated with using surgical screws.
Further, it would similarly be an improvement over the prior art to provide apparatus and methods for reliably implanting an acetabular cup that will result in a reliable implant, but which avoids the complications associated with using surgical pegs.
It would yet be a further advancement in the art if such apparatus and methods for implanting a prosthetic acetabular cup resulted in a reliable implant but which also included a way to test the stability of the implant to allow the surgeon the option of using secondary securement means known in the art if necessary.
Finally, it would yet be an even greater improvement over the prior art if such implantation and testing methods could be carried out with a minimal amount of equipment and in a minimum amount of time.
Such apparatus and methods are disclosed herein.